nuageinit.7: fix cloud-config header line

This updates the nuageinit man page by aligning mentions of the
cloud-config header line with the cloud-init documentation[0], removing
an unwanted "!" character.

[0] https://docs.cloud-init.io/en/latest/explanation/about-cloud-config.html#how-do-i-create-a-cloud-config-file

Signed-off-by: Jonathan Matthews <freebsd@hello.jonathanmatthews.com>
Differential Revision: https://reviews.freebsd.org/D53706